The present invention relates generally to carbon-containing refractory elements which are subject to thermo-chemical attacks in atmospheric air, and more specifically to wearable carbon-containing parts of a slide valve for use in controlling flow of molten metal from a metallurgical vessel.
Various refractory elements, such as molded bodies and wearable slide valve parts, include carbon in the refractory material. These refractory elements can be formed by various methods including molding and compression techniques. The carbon contained in such refractory elements may be included with the refractory material as, for example, a material fraction, a component of a binder, or an impregnating agent. When such elements are used at elevated temperatures, some degree of decarburization will invariably take place whenever the element is exposed to oxygen. Such decarburization generally begins at about 400.degree. C. and becomes quite pronounced at about 600.degree. C. The decarburization begins at the outer surfaces of the element which are exposed to the ambient air (or other oxidizing medium) and, as the air penetrates into the pores vacated by the carbon, takes place increasingly deeper within the refractory element. As the decarburization moves inwardly from the outer exposed surfaces, it increases the porosity of the refractory element, thereby decreasing the strength and stability thereof. This leads to premature wear of the refractory element, and will eventually destroy the element for its intended purpose.
When the carbon-containing refractory elements are wearable parts of a slide valve for use with a metallurgical vessel, they are, in addition to being subjected to decarburization, subjected to the normal chemical erosion and thermal shock and/or abrasive wear caused by contact with molten metal at high operating temperatures of the slide valve. This contributes to the wear of the refractory elements and results in an even quicker destruction of the elements for their intended purpose. The wearable refractory parts which are utilized in the slide valve can be, for example, inlet sleeves, bottom plates, slide plates, and/or outlet sleeves. Due to the conditions in which these parts are used, including the chemical and abrasive wear caused by contact with molten metal at high operating temperatures and the decarburization discussed above, these parts must be replaced frequently.
In order to inhibit the chemical and abrasive wear of the wearable slide valve parts due to contact with the molten metal, it is known to impregnate the refractory parts with an impregnating agent such as tar or the like. During operation of the slide valve, the tar impregnated in the refractory parts migrates toward wearable surfaces (i.e. surfaces subjected to wear due to contact with the molten metal) of the parts and acts as a lubricating agent and to otherwise protect the wearable surfaces. However, the tar migrates toward not only the wearable surfaces, but also toward the surfaces which are exposed to the ambient air. From these exposed surfaces, the tar evaporates and otherwise escapes. The escaped tar creates environmental pollution and also contaminates the slide valve and its operating equipment.
It has been proposed in DE 3921 794.9 and corresponding U.S. application Ser. No. 07/547,149, filed July 3, 1990, to provide refractory elements of a slide valve with a gas impermeable thermally stable sealing layer over the surfaces thereof which are exposed to the ambient air.
It has also been proposed in DE 33 13 015 A1, which corresponds to U.S. Pat. Nos. 4,617,232 and 4,621,017, to provide an article formed of graphite or other carbon material with a protective layer of silicon carbide (SiC) so as to protect against corrosion and oxidation at high temperatures. It is also disclosed in these documents that it has been suggested to first coat the graphite article with silicon carbide or silicon oxicarbide and then additionally coat it with a glaze chosen from among oxides of boron, silicon, aluminum, phosphorus, magnesium, calcium and zirconium, so as to avoid cracking of the SiC coating when subject to thermal shock. However, these documents do not suggest the use of a boron-based impregnating agent alone to protect against corrosion and oxidation. Additionally, these documents do not suggest use of those coatings on tar-containing or tar-saturated refractory elements. Also, the process of coating the graphite article is an expensive multiple step process.